I don't get why people think that bitcoin is a ponzi scheme/pyramid scheme.
There is simply no central entity, and that is the exact thing that bitcoin is based on. There isn't anyone that is collecting investments. When you use bitcoin, there isn't any single person that benefits from it. That's the plain truth.
Also, it's important to note that bitcoin doesn't even promise any returns. Its price fluctuates due to market supply and demand. How this can be classified as a ponzi scheme is really just beyond me.
I'd say that most people would agree nowadays that bitcoin is a legitimate currency, though it was disputed in the past.
The short answer is because it is. It doesn't matter if there is no central entity to gain most profits (if we don't consider Satoshi as such, of course), but that doesn't take anything from what any pyramid scheme is all about. They are all about collecting money from newbies and distributing it among old members. When you buy coins, you have to buy them from someone else, and if you want to make money, you should sell them to someone else at a profit (and in most these will be two different someones). If you don't like the phrase, you can call it a zero-sum game, but it is essentially the same old soup heated again.
